1. Field of the Invention
The invention pertains generally to the field of blades for fans and propellers. A preferred and advantageous application of the invention pertains to the use of the invention in bidirectional marine thrusters.
2. Prior Art
Traditional blades for propellers or fans have either been oriented on the hub with the longitudinal axis of the blade extending in a substantially straight radial direction from the hub, or more recently, radially skewed such that the leading edge of the blade presents a curved front to the fluid. The skewed blade provides many advantages over the straight blade. These advantages include less noise and less cavitation due to decreased velocity components normal to the leading and trailing edges as the blade travels through the fluid. This advantage can be understood by reference to the velocity vector diagram shown in FIG. 2. As the hub 100 rotates in a clockwise direction as indicated by direction of rotation arrow 101, the relative velocity of the fluid impinging the leading edge of straight blade 103 is V.sub.R. With a constant rotational velocity, the relative velocity of the fluid V.sub.R will increase as the distance from the center of the hub increases. With a blade skewed for clockwise rotation 102 the curve of the leading edge of the blade can be skewed such that the normal velocity of the fluid impinging the blade edge V.sub.R sin (theta) is constant or is lower by the factor sin (theta). In other words the relative velocity V.sub.R impinges the blade 102 at an angle (theta) so that its component perpendicular to the leading edge is V.sub.R sin (theta). This makes the blade react as if it was in a flow velocity V.sub.R sin (theta) rather than V.sub.R with a straight blade 103.
This skewing greatly reduces the underpressures on the blade causing resistance to cavitation and noise. Unfortunately, with the traditional skewed blade, these advantages are achieved only as the result of a loss of stiffness and strength. The skewed propeller is limber, flexible and more easily deformed by lift loads. As is apparent these disadvantages are amplified when the skewed blade 102 is rotated in the opposite direction. Thus, the advantages of the skewed blades have heretofore been constrained to unidirectional applications. The present invention yields all of the advantages of the skewed blades without the disadvantages.